The need for light weight ablative materials for aerospace and military applications is well established. Petroleum and coal tar pitches are currently used as carbon matrix precursors in the fabrication of carbon-carbon composites. The Air Force baseline, high density composite matrix precursor, has been ALLIED CHEMICAL COMPANY's 15 V coal tar pitch. While this material has provided acceptable properties in carbon-carbon composites, there are disadvantages in its use. Some of these are:
1. variable composition and properties;
2. difficulty in processing;
3. impurities which are variable in quantity and composition;
4. low char yield, in low pressure processing; and
5. uncertainty in future availability.
The variable composition of the 15 V pitch is the result of its derivation from a fossil fuel which is variable both in its organic origin and in its final state. The inorganic ash impurity content also is highly variable.
The lack of constancy of pitch compositions in general is reflected in variations in properties such as viscosity, char yield, gaseous decomposition products, and the nature of the char, such as porosity, hardness, and morphology. Attempts to improve processibility of coal tar pitches by pretreatment procedures have been helpful, but have not solved all of the problems indicated.
Ash content variations of pitches, such as 15 V, very likely affect such factors in a composite as matrix crystallinity, porosity, and strength as well as the char yield. The quinoline insoluble fraction of pitches (QI's) varies widely and inorganic substances affect ablation performance adversely through the char structure, conversion of char to graphite, or by catalyzing the oxidation of the graphite in air.
In addition to the petroleum and coal tar pitches, aromatic heterocyclic polymers have also been available in recent years. They include both condensation polymers such as the polybenzimidazoles, pyrrones, polyimides, polyquinoxalines, addition type polymers such as acetylene-substituted polyimides (for example Thermid 600 or HR600), acetylene-terminated quinoxaline (ATQ), bismaleimides (P13N and P105A), Michael-addition type polyimides (Kerimide) and "polymerized monomeric reactants" (PMR).
Several of these polymers are good char formers after they are fully cured, but those that cure by condensation evolve large volumes of gaseous by-products during the cure. Furthermore, since these materials are not processable after cure, (for example polyimides, PBI, pyrrone) they must be processed in their prepolymeric outgassing form. In addition, volatile solvents are required during processing. As a consequence, if used to produce 3-D carbon-carbon structures, these latter polymers liberate large volumes of gases before and during pyrolysis, thus yielding very porous structures.
Addition-type polymers such as HR600, P13N, and P105A are theoretically better suited for the 3D carbon-carbon applications, but their melting points are too close to their cure temperatures and thus they do not remain fluid for a sufficiently long period of time at their melting points to allow them to be effectively processed in the 3-D carbon-carbon applications.
The closest art to the present invention known to the inventors involves the use of prepolymers of diethynylbenzene. One of these materials was sold as "H resin" by Hercules Chemical Co. of Wilmington, Del., although it has since been removed from the market. "H resin" was sold as a high char forming laminating resin, however, its cure was too difficult to control. "H resin" was a solid polymer rather than a low melting low viscosity material. Thus, it was sold as a polymer solution. In this form, when used to impregnate a woven 3-D graphite fabric, all of the solvent could not be removed, and thus pyrolysis yielded a very porous structure. It thus was not suitable for use as an impregnant. It should be noted that diethynylbenzene, from which "H resin" is made, can polymerize explosively when heated. Thus, its use as a homopolymerizable liquid impregnant is highly unlikely.